Industry is constantly searching for lighter, stronger, and more resistant materials to be used in place of the materials used today. Cyanate ester resins are known for their thermal stability, chemical inertness, solvent resistance, and electrical properties. Thus, more and more uses are being found in a variety of fields which demand high performance materials, such as structural composites, printed wiring boards, semiconductor encapsulants, structural adhesives, injection molding and prepregs, and high performance binders. The high performance characteristics of cyanate ester resins are offset by their brittleness. To expand their utility, several strategies have been pursued to toughen these materials.
Several patents have dealt with curable compositions comprising cyanate ester monomers and ethylenically unsaturated monomers. See, for example, U.S. Pat. Nos. 4,600,760, 4,116,946, and 4,383,903. Catalysts used for cyanate cure are metal salts, such as zinc octoate, cobalt naphthanate, or certain amines. Use of organometallic catalysts was not taught or suggested. No control over morphology was taught in any of the references.
Cyanate ester resins are formed from polyfunctional cyanate monomers. The use of an organometallic compound as thermal and/or photocatalyst for the cure of a cyanate ester resin has been described in U.S. Pat. No. 5,215,860.
Polymerization of ethylenically unsaturated monomers by thermally or photochemically generated free radicals is well known in the art (see J. M. G. Cowie in "Comprehensive Polymer Science", G. Allen and J. C. Bevington, Eds., Pergamon Press, Oxford, 1989, Vol 3, pages 1-15). Typical free radical generators include organic peroxides, onium salts, azo compounds, and carbonyl compounds.
The use of photoreactive organometallic transition metal carbonyl complexes in conjunction with organic compounds, such as organohalo compounds (H.M. Wagner and M.D. Purbrick, J. Photographic Sci. 1981, 29, 230) and electron accepting olefins (C. H. Bamford and S. U. Mullik, J. Chem. Soc. Faraday I 1976, 72, 368), in free radical curing is known. For example, benzenechromium tricarbonyl with CCl.sub.4, has been used to photopolymerize methyl methacrylate (C. H. Bamford and K. G. Al-Lamee, J. Chem. Soc. Faraday I 1984, 80, 2175) and styrene (C. H. Bamford and K. G. Al-Lamee, J. Chem. Soc. Faraday I 1984, 80, 2187); in both cases, the active initiating species has been shown to be the CCl.sub.3 radical and little or no curing occurs in the absence of CCl.sub.4.
Gatechair et al. (U.S. Pat. No. 4,707,432) has shown the combination of certain cationic organometallic complexes, such as ferrocenium salts, with .alpha.-cleavage photoinitiators, such as acetophenone, to be useful photoinitiator systems for free radical polymerizations. DeVoe and Palazzotto (EPO Publication No. 0 344 911 A2) have further shown that certain cationic organometallic complexes by themselves are useful photoinitiators for acrylic polymerizations.
Cyanate ester resins have been proposed for use in vibration damping. U.S. Pat. No. 4,223,073 describes the use of polycyanurates in "high temperature damping" composites. These are single component systems in that cyanate ester groups are the only polymerizable groups present; thus, little control is possible over morphologies of the cured resin. Damping properties are varied by changing the organic backbones of the polycyanurates and the effective dampers are not based on commercially available resins. Further, the "high temperature damping" appears to refer to temperatures of around 100.degree. C. Japanese Patent Applications 4,202,316, 4,202,353 and 4,202,354 all describe vibration damping materials comprising preformed saturated polyesters and cyanate ester monomers.
U.S. Pat. No. 3,833,404 discloses a viscoelastic layer for use in a damping means for a vibratory part, the layer being an interpenetrating polymeric network (IPN) which is broadly stated to consist essentially of an elastomer and a plastic. The IPN is prepared sequentially where the first component is cured in the absence of the second component which is then added by swelling or combining the two components as latexes. There is no disclosure to simultaneous or sequential formation of the networks in the presence of both components. The networks are independently crosslinked and continuous.
Electronic adhesives based on cyanate ester compositions cured with organometallic catalysts have been described in U.S. Pat. No. 5,143,785. Cyanate esters are combined with preformed thermoplastic polymers, conductive particles, catalysts and coupling agents in a solvent such as tetrahydrofuran and preferably coated on a release liner to provide an adhesive film. Only the cyanate ester component is cured by the organometallic catalyst because the other component is prepolymerized.